How did prejudice and discrimination contribute to the collapse of the group?

Overall, male against female discrimination contributes to
Okonkwo's tragedy, but I don't think it contributes to the collapse of the tribe.  White
against black racism and Christian monotheism against tribal polytheism contribute more
toward the collective unraveling of culture.

In Part I of
Things Fall Apart, the Igbo tribe is segregated in terms of gender
roles.  Men farm for yams; women cultivate cassavas and beans.  Men take part in the
wrestling and masked dancing; the women prepare for these
feasts.

Whether this is gender discrimination or sexism
depends on which culture is framing the question.  Our independent modern culture, which
champions integrated gender roles and feminism, would certainly call this male prejudice
against women.  Specifically, Okonkwo has rigid roles that he feels he should play, as
well as his wives and male and female children.  For example, Nwoye acts a bit feminine
and Enzimna acts a bit masculine, but Okonkwo makes sure that neither crosses over into
the gender duties of the other.  These roles contribute to Okonkwo's fear of being weak,
which--in turn--leads to his exile and eventual suicide.  These differences alone,
though, do not cause the group to fall apart.  In short, they tolerate what we call
sexism.

Religion is a major unraveling agent in Parts II
and III.  The white Christian colonists feel that the Igbo are polytheistic and,
therefore, pagan.  Little do the British know that the tribe has one major god and many
other gods doing his service.  Conversely, the tribe has little religious prejudice
against the Christians.  At first, the Igbo largely ignore the Christians because they
have set up camp in the evil forest and, as a result, the Igbo feel the missionaries are
going to be obliterated.  The survival of the church and the converts obtained are a
major contributing factor in the tribal split.  The Christians expose the tribe's
practice of killing twins and cruelty to
outcasts.

Racially, there is some white against black
racism, but it is not as developed as the religious differences.